In U.S. Pat. No. 3,528,950 a fully imidized prepolymer having reactive end groups is disclosed. In this disclosure, the water of imidization is removed before final cure of the prepolymer during fabrication of the polyimide product, thus solving the prior problem of void formation from water vaporization. However, solvent solubility was not as desirable as many fabricators would prefer.
Subsequently, U.S. Pat. No. 3,812,159 taught that a dianhydride monomer containing a phenoxyphenyl sulfone linkage could be used in the process taught by U.S. Pat. No. 3,528,950, and which would provide polyimides with improved solubility. The characteristics and synthesis methods for these polyimides are taught in U.S. Pat. No. 3,699,073.
While U.S. Pat. No. 3,812,159 solves the solubility problem, the high temperature stability of the sulfone-containing polyimide is not satisfactory. U.S. Pat. No. 4,203,922 seeks to improve the chemical and thermal stability of polyimides by incorporating an fluorine-containing aromatic diamine compound into the polymeric chain while maintaining their solubility characteristics. The compound may be characterized by the following formula: ##STR1## This compound is synthesized by an aromatic nucleophilic substitution of the chloro group on 4-chloronitrobenzene with a phenoxide ion.
The above reaction is initiated by reacting stoichiometric amounts of 2,2-bis(4-hydroxyphenyl)hexafluoropropane with sodium hydroxide to produce the disodium salt according to the following equation: ##STR2## This reaction produces an intermediate compound which is sufficiently active to enter into a nucleophilic displacement reaction with the chloro-substituent on 4-chloronitrobenzene to produce 2,2-bis[4-(4-nitrophenoxy)phenyl]hexafluoropropane, as follows: ##STR3## Reduction of the dinitro compound to the corresponding diamine is accomplished by reacting the compound with water in the presence of iron, stannous chloride, or reduction with hydrogen with palladium as a catalyst.
The reaction to form the sodium alkoxide is difficult to control because of the occurrence of a variety of side reactions. Sodium hydroxide typically attacks the solvent in addition to the bisphenol compound. A monosodium intermediate is also formed in the first (sodium hydroxide) step leading to: ##STR4## and the corresponding monoamine on hydrogenation. The end result of this myriad of competing reactions is that the yield of desired diamine is severely depressed and its purity is adversely affected.
It is an object of this invention to provide a reaction sequence where 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane can be produced with a minimum of side reactions.
It is a further object of the present invention to provide a process for the preparation of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane that is of high purity.